Drills of the above-noted type are previously known from U.S. Pat. No. 4,983,079 and U.S. Pat. No. 5,088,863. With these prior art drills, however, clearance surfaces at the edges extend in an unbroken manner from the peripheral end of the edge until its curved portion adjacent the core centering portion. This means that the drill head obtains a tip angle only in one main plane that is parallel with the straight edge portion but not in a plane that extends laterally from the main plane. Despite providing a core centering portion, such drills can perform with only a marginal improvement of its centering ability compared with conventional drills which are not provided with such centering core portions. An essentially similar drill is disclosed in U.S. Pat. No. 4,583,888.